How do biocides interact with bacterial membranes to disinfect?

The growing Hospital Acquired Infections as well as Covid infections transmitted from contaminated solid surfaces over the past few years have challenged our ability to fight harmful microorganisms effectively. Cationic surfactants (quaternary nitrogen compound or abbreviated as quats) are one of the most widely used disinfectants. Despite their intensive use, little is actually known about how cationic surfactants interact with microbial membranes and kill the pathogens in the presence of other additives, e.g. hard water ions, nonionic surfactants. This situation limits our ability to design and formulate effective disinfectants. Weak or ineffective disinfectants, when used in hard surface cleaning (surgical devices, beds and airing systems and even food and diary processing facilities), could lead to outbreaks that could cost millions and, in some cases, have casualties. A formulated cationic quat disinfectant cleaning product often contains nonionic surfactants to provide cleaning efficacy. Despite decades of research and development, little is known about the role of the nonionics in disinfection. This lack of understanding creates a vacuum when new products are developed. Without this vital information about the interfacial biocidal action of such blends it is difficult to balance the levels of quats in product formulation.

Biocidal quat molecules can bind (and usually do) to microbial membranes via strong electrostatic attraction and kill pathogens by structural disruption and membrane leakage. This mode of action is well supported by the rugged or disrupted surfaces of bacteria and fungi viewed from imaging studies such as scanning electron microscopy and confocal microscopy. Membrane disruptions have also been monitored by membrane permeation probes and fluorescence detection, zeta potential measurements and dynamic light scattering using lipid membrane models, such as spread lipid monolayer, supported lipid bilayer and small unilamellar vesicles (SUVs). High consistency to real microbial measurements validates the model membrane approaches. However, current techniques do not have the sensitivity or resolution to structural changes within the membrane which is typically in the region 1-5 nm. Lack of capability to follow structural changes during a biocide binding makes it difficult to distinguish one biocide from another or explore the impacts of different membranes.

Neutron reflection (NR) and scattering (SANS) are about the only techniques that offer the insights into the structural changes across lipid membranes upon quat binding, with and without nonionic surfactant. Successful demonstration of the neutron experiments in this exploratory project replies on (a) input of neutron expertise in the running of the neutron experiments, synthesis of the deuterated surfactants and data analysis and interpretation from the ISIS team, (b) the expertise of antimicrobial work and selection of model lipid membranes from the Manchester team and (c) the active participation of Arxada in relating model interfacial studies to real quat formulations. The project teams have worked together to devise the challenging workplan that will be delivered by a highly able PDRA, Dr Mingrui Liao, who has already had prior knowledge of the quat biocides in his current PDRA work and neutron experiments from his PhD research on antimicrobial peptides.

Successful outcomes from this project will form a strong basis for the collaborating teams to send joint grant applications to BBSRC and EU to engage in this new area of research by seeking more systematic neutron experiments. The teams will also publish their results in leading international journals such as JACS and Nat Commun.